1. Field of the Invention
This invention relates to the heating of materials based on oxides of silicon and/or germanium such as SiO.sub.2 and GeO.sub.2 and is particularly useful in the manufacture of semiconductor devices.
2. Description of the Prior Art
In the manufacture of semiconductor devices, phosphosilicate glass is often used to form a protective coating on partially completed devices. To prevent cracks from forming in conductive leads or contacts to be formed on the surface of this coating, abrupt changes in the height of this coating must be eliminated. This is usually done by reheating the glass coating to "flow" the glass so as to smooth out surface discontinuities.
The phosphosilicate glass "flow" or "reflow" process both rounds off sharp edges of etched cuts on phosphosilicate glass to help prevent cracking at these edges of subsequently deposited metal or polycrystalline silicon and smooths the overall surface of a layer of phosphosilicate glass to enhance the subsequent controlled etching thereof. The flow is normally induced by a high temperature (approximately 950.degree. C. or above) furnace anneal, for approximately 30 to 60 minutes.
In general, the phosphosilicate glass layer is part of a structure initially formed by providing a silicon substrate with a thermally deposited silicon dioxide (SiO.sub.2) layer thereon. A layer of P.sub.2 O.sub.5 --SiO.sub.2 is then deposited on the layer of thermally deposited SiO.sub.2 by, for example, chemical vapor deposition (CVD) involving the reaction of a phosphorus-silane mixture and oxygen at low temperature (i.e. about 400.degree. C.), so that a top layer of phosphosilicate glass is provided. Such a CVD process is described, for example, in an article by Kern, et al. entitled "CVD Glass Films for Passivation of Silicon Devices: Preparation, Composition and Stress Properties", appearing in RCA Review, Vol. 37, Mar. 1976, pages 3-54, which is hereby incorporated by reference.
A major problem with the prior art approach of inducing flow by high temperature anneal is that when semiconductor device structures are heated to a relatively high temperature in the furnace, the properties of these structures often undesirably change due to dopant diffusion, alloying and contamination. For example, aluminum alloys with silicon at 577.degree. C. and melts at 660.degree. C. Thus, any reflow performed at temperatures in excess of these temperatures will destroy or alter any aluminum present in the device. The diffusion of dopants at high temperatures (e.g. temperatures above 900.degree. C.) is well known. Another problem is that dopants, such as P.sub.2 O.sub.5 typically in the concentration of 7-9 mole percent, must be added to the SiO.sub.2 base composition to lower the flow temperature. The presence of phosphorus in the glass subsequently enhances corrosion of commonly-used aluminum interconnections.
The use in semiconductor devices of glass layers comprising germanium oxide (GeO.sub.2) has been disclosed in U.S. patent applications Ser. No. 243,987 filed Mar. 16, 1981, Ser. No. 243,988 filed Mar. 16, 1981, and Ser. No. 243,989 filed Mar. 16, 1981, which are assigned to Fairchild Camera & Instrument Corporation, the assignee of this application. These three copending applications are hereby incorporated by reference. The use in semiconductor devices of glasses including an oxide of germanium such as GeO.sub.2 is advantageous due to the fact that such glasses flow at lower temperatures than do glasses comprised solely of SiO.sub.2.